In most concrete and steel structures accomodation must be made for relative movement between structural members in order to avoid the build-up of dangerous stresses. The source of this movement may be:
- Thermal expansion and contraction
- Permanent creep and shrinkage
- Post tensioning strain
- Live load deflections
- Earth movement
It is usually desirable to minimise the resistance forces and moments resulting from these movements and this is the primary function of Structural Bearings.
Bearing Technologies range of Pot Bearings provide this function by the most efficient method using well proven concepts combined with modern manufacturing technology and advanced materials.
This is achieved by exploiting the unique low-friction properties of PTFE (polytetrafluoroethylene) and the semi-hydraulic behaviour of restrained elastomers.
PTFE is a well known polymer which possesses some remarkable properties. The coefficient of friction when sliding against a polished surface can be as low as 0.01 which compares with ice sliding on ice. In addition, its coefficient of friction is the lowest at highest specific pressures which make it particularly suitable for use in structural bearings.
Glacier DU is a proprietary composite material comprising a PFTE mixture impregnated into a metallic structure. It is capable of withstanding extreme pressures whilst possessing similar friction properties to those of pure PTFE. It is used in Bearing Technologies Pot Bearings for highly stressed components.
In order to minimise space requirements, Pot Bearings are designed for relatively high contact pressures against the structure. This also ensures lowest friction values. Most elastomers cannot sustain such pressures in their free state and must be effectively contained to prevent overstraining. When constrained in this way, the elastomer behaves as a semi-viscous fluid and can safely accomodate useful degrees of angular displacement. All Bearing Technologies Pot Bearings can safely accomodate 0.01 radians rotation about any axis.
Correct installation of all types of structural bearings is critical to their performance. The most important aspects of installation are:
- Ensuring intimate surface contact with the structure-there must be absolutely no voids above or below the bearings.
- Cementatious or epoxy grout must be properly mixed and must be of adequate strength.
- Bearing orientation and presets must be carefully checked and adjusted where necessary.
- Horizontal and vertical alignment.
- Where applicable, anchor bars must be substantially encased within the steel reinforcement of concrete members.
- Prevention of contamination, especially slurry, of the exposed stainless steel sliding surfaces.
- Preventing mechanical damage to the corrosion protection system.
BT Pot Bearings are designed for 0.01 radians rotation about any axis. When rotation occurs the moment of resistance from the rubber disc is transferred to the structure. The value of this moment at 20°C :
Mr = 4.5×10– x AA kN-m where AA is from the tables pages 3-5
This moment results in an increase in concrete pressure at one edge of the bearing and a corresponding decrease at the opposite edge:
The maximum value is Δfc = 4.8 Mpa. This value is reduced proportionately with lower rotation angles.
The coefficient of friction of the sliding surfaces can be assumed to vary between 0.015 to 0.05 depending on a number of operating variables. The most important is specific pressure. The graph below illustrates this relationship. It is safe, however, to assume a C of F of 0.03 under maximum load conditions.
© GLACIER DU is the registered trademark of Dana Inc